1. Field of the Invention
This invention relates to methods for relaying information between two or more points using laser or optically generated conductive transport means through the atmosphere to convey the information.
2. Description of the Prior Art
Radio waves or other optical transport waves and less sophisticated systems using wires, fiber-optic cables, and other physical transport means have been employed to relay information between two or more points. Radio transmissions are heavily dependent upon atmospheric conditions, and fiber-optic cables or other physical transport means are expensive.
Communication systems are therefore needed that do not rely upon radio transmissions, fiber-optic cables, or other physical transport means.
An open path laser beam communication system overcomes many of the problems associated with prior art communication systems, but has its own limitations. Specifically, meaningful multi-channel WDM (Wavelength Division Multiplexed), open-path laser communication requires that each laser be at a different wavelength, and possibly tunable. Tunable lasers require tuning elements such as gratings, etalons, and the like. These tuning elements can tune wavelengths to a precision of 1 nm (1 cm−1). Finer wavelength control is difficult and expensive to maintain using conventional techniques. Typically, a laser wavelength is locked to a peak maximum absorption of a gas that is internal or external to the laser cavity, or to an external wavelength spectrometer instrument.
Wavelength-controlled laser/optical open-path communication systems often have difficulty in controlling the laser wavelength to that of the receiver optical bandwidth. Moreover, multi-wavelength channel cross talk is often a problem because it is difficult to produce very narrow (<10 nm) optical filters.
However, in view of the prior art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the art how the limitations of the known systems and methods could be overcome.